/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
class Solution {
public:
  TreeNode* invertTree(TreeNode* root) {

    // 如果结点不为空
    if(root){
      swap(root->left, root->right);
      invertTree(root->left);
      invertTree(root->right);
    }

    return root;

  }
};